This invention relates to apparatus for producing helically wound double-walled thermoplastic pipes having a corrugated outer wall and a smooth inner wall, both the walls being fused together.
Known such apparatus (for example, Canadian patent No. 1,194,662 of the present inventor and U.S. Pat. No. 4,575,400 of Ueda et al.) comprise the following components:
extruder means communicating respectively with first and second dies having extrusion orifices for extruding accordingly the first and second bands of a thermoplastic material for forming appropriately the outer and inner walls of the produced double-walled pipe;
a former located between said dies, downstream of the first die and upstream of the second die, their extrusion orifices being situated substantially along the former, the same having a lateral surface adapted to receive the first extruded band, to shape the corrugated configuration of the latter and to helically wind the same into the outer corrugated tube of the produced pipe, the inside of said outer tube receiving the second band helically wound into the inner smooth tube of the pipe;
an elongate tubing mounted inside the former and connecting the second die with its extruder means;
a cooled pressing element conforming to the inner wall and engaging the second band as it emerges from the second die and pressing the same to the outer wall to fuse them together.
The main difference between the known apparatus lies in the structure of their formers. In the first patent, the former is represented by a mandrel having an endless helical lateral surface. In the second patent, the former is represented by a plurality of rotating rollers arranged in a circular array.
The known devices have the followiong drawbacks:
fusing still unstable and soft plastic strip of the inner tube to the already formed outer tube wherein the inner tube turns from a soft to a rigid stage;
retraction of the band from the pressing element;
nonuniform change of the internal pressure of the strip.
These drawbacks lead to imperfections of the inner tube, such as shrink marks (depressions of its surface) in the spots of the retractions, warpage (caused by said change) and wrinkles having appearance of a wave.